I ran into a bit of an issue in Rails Cast #142. I received the following error while testing the IPN:
WARNING: Can't verify CSRF token authenticity Completed 500 Internal Server Error in 2ms app/controllers/payment_notifications_controller.rb:11:in `process_ipn_payment'
Simply replace the following line with the alternative below it:
protect_from_forgery :except => [:create]
skip_before_filter :verify_authenticity_token, :only => [:create]